Protein kinases specific for tyrosine have been associated with the control of cell growth. A new antiserum specific for phosphotyrosine was developed to identify and purify phosphotyrosine-containing proteins. Using this antiserum, I propose to (1) analyze a large number of cell lines (emphasizing human tumor lines) for phosphotyrosine-proteins, (2) measure the response of the same set of cell lines to epidermal growth factor (EGF), (3) isolate and characterize a newly identified phosphoprotein which is involved in the response to EGF and (4) identify any phosphotyrosine-proteins in cells treated with tumor promotors, sarcoma growth factor and a number of other substances related to carcinogenesis, cell growth and stimulation.